Tetraalkylcyclobutane-1,3-diols can be important intermediates for producing a variety of polymeric materials which possess advantageous properties. For example, polyesters derived from dicarboxylic acids and 2,2,4,4-tetramethylcyclobutane-1,3-diol possess higher glass transition temperatures, superior weatherability, and hydrolytic stability compared to polyesters prepared from other commonly-used diols. Tetraalkylcyclobutane-1,3-diols can be prepared by the catalytic hydrogenation of the corresponding 2,2,4,4-tetraalkylcyclobutane-1,3-dione (I) to the corresponding 2,2,4,4-tetraalkylcyclobutane-1,3-diol (II), wherein R is an alkyl group, as illustrated in FIG. 1.
The hydrogenation of the corresponding tetraalkylcyclobutanediones can be carried out using a variety of catalysts such as, for example, nickel, ruthenium, and cobalt. For example, the hydrogenation of 2,2,4,4-tetramethylcyclobutane-1,3-dione to 2,2,4,4-tetramethylcyclobutane-1,3-diol can be accomplished using nickel or ruthenium containing catalysts as described in U.S. Pat. Nos. 3,000,906, 3,190,928; 5,169,994; 5,258,556; and 2,936,324. Cobalt containing catalysts also can be used. For example, U.S. Pat. Nos. 5,528,556 and 5,169,994 disclose that Raney cobalt is effective for hydrogenation of 2,2,4,4-tetramethylcyclobutane-1,3-dione to 2,2,4,4-tetramethylcyclobutane-1,3-diol. These processes, however, can exhibit long reaction times. The reaction rate can be increased by raising the temperature and pressure, but these measures can be expensive and can result in the formation of by-products from the decomposition of the products and starting materials.
In general, the hydrogenation of 2,2,4,4-tetramethylcyclobutane-1,3-dione produces the corresponding 2,2,4,4-tetramethylcyclobutane-1,3-diol as a mixture of cis and trans isomers. For example, U.S. Pat. No. 3,190,928 discloses a process for hydrogenation of 2,2,4,4-tetramethylcyclobutane-1,3-dione to 2,2,4,4-tetramethylcyclobutane-1,3-diol using nickel- or ruthenium-based catalysts that produce cis:trans molar ratios ranging from about 0.5:1 to about 1.2:1 depending on reaction conditions. A wide variation in the cis:trans ratio, however, can result in polyesters with inconsistent and/or undesirable properties. In addition, for some applications, a high cis:trans ratio is desirable, but catalysts that produce a high ratio of cis:trans isomers may not give the good yields or acceptable rates of hydrogenation.
There is a need in the art, therefore, for a process that can produce 2,2,4,4-tetramethylcyclobutanediol at good conversions and selectivities and with consistently high cis:trans isomer ratios, e.g., greater than 1:1. There is also a need for such a process that can be operated economically at modest pressures and temperatures.